<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div {
            width: 100px;
            height: 100px;
            position: relative;
        }
        .a {
            background-color: black;
            top:20px;

            z-index: 10;
        }
        .b {
            background-color: blue;
            top:-40px;
            z-index: 12;
        }
        .c {
            background-color: red;
            top:-60px;

            z-index: 11;
        }
    </style>
</head>
<body>
<div class="a"></div>
<div class="b"></div>
<div class="c"></div>
</body>
</html>